Cup holders United were the clear favourites going into the tie against the struggling Hammers at a snow-bound Upton Park but the home side made light of the teams’ league standing with a convincing win.
Former Red Devil Jonathan Spector scored two in the first half – his first ever goals in English football – while striker Carlton Cole also struck twice in the second spell to send West Ham to the League Cup semi-finals for the first time in 20 years.
Reid, who appeared in the opening few games of the season but has since lost his place in the starting line-up, came on as a substitute in the 74th minute to replace fellow defender James Tomkins.
Making it on to the pitch for such an occasion is something for the 22-year-old to put in his scrapbook as the upset is West Ham’s biggest win over United in 80 years.
He will now look to break back into manager Avram Grant’s side in the premier league as the Hammers look to improve their position at the foot of the table.
Reid, who spent his younger years on Auckland’s North Shore and is of Maori descent, has made three starts in this premier league so far but was forced out of the team when the experienced Lars Jacobsen was signed.
The 22-year-old came to the attention of the football world during the All Whites’ remarkable undefeated campaign at the FIFA World Cup in South Africa with a last-gasp goal against Slovakia and another couple of assured performances in further draws against Italy and Paraguay.